The invention relates to catalytic cracking of hydrocarbons. Particularly the invention relates to a method providing improved selectivity for cracking hydrocarbon feedstocks to propylene by contacting the hydrocarbon under cracking conditions with a catalyst selected from the non-zeolitic molecular sieves consisting of silicoaluminophosphates (xe2x80x9cSAPOxe2x80x9d), metal aluminophosphates (xe2x80x9cMeAPOxe2x80x9d), metal aluminosilicophosphates (xe2x80x9cMeASPOxe2x80x9d) elemental aluminophosphates (xe2x80x9cElAPOxe2x80x9d) and elemental aluminosilicophosphates (xe2x80x9cElASPOxe2x80x9d) where the metals include divalent Co, Fe, Mg, Mn, and Zn and trivalent Fe and the elements include Li, Be, B, Ga, Ge, As, and Ti.
Thermal and catalytic conversion of hydrocarbons to olefins is an important industrial process producing millions of pounds of olefins each year. Because of the large volume of production, small improvements in operating efficiency translate into significant profits. Catalysts play an important role in more selective conversion of hydrocarbons to olefins.
While important catalysts are found among the natural and synthetic zeolites, it has also been recognized that non-zeolitic molecular sieves such as silicoaluminophosphates (SAPO) including those described in U.S. Pat. No. 4,440,871 also provide excellent catalysts for cracking to selectively produce light hydrocarbons and olefins. The SAPO molecular sieve has a network of AlO4, SiO4, and PO4 tetrahedra linked by oxygen atoms. The negative charge in the network is balanced by the inclusion of exchangeable protons or cations such as alkali or alkaline earth metal ions. The interstitial spaces or channels formed by the crystalline network enables SAPOs to be used as molecular sieves in separation processes and in catalysis. There are a large number of known SAPO structures. The synthesis and catalytic activity of the SAPO catalysts are disclosed in U.S. Pat. No. 4,440,871.
In other crystalline microporous solids belonging to the class of aluminophosphates the framework is normally neutral (Al (III):P (V) atomic ratio =1). This framework can be made negative and thereby gives these materials advantageous properties such as adsorption, cation exchange or catalytic activity by replacing P(V) or the pair Al (III), P (V) with a tetravalent element such as silicon, converting to the closely related SAPO structure discussed above, or by replacing Al (III) with a metal, especially a divalent metal such as zinc or cobalt, the materials obtained being denoted by the acrornym MeAPO where Me is the metal, or else by combining these two types of substitution the materials obtained being denoted by the acronym MeAPSO. A group of such materials is described in U.S. Pat. No. 5,675,050.
In the International Application WO 91/18851 that exchange of cations to provide Lewis acid sites in zeolite and SAPO catalytic structures in isomerization catalysts is disclosed. SAPO-11 is disclosed as being particularly effective in this system. The application focuses on skeletal isomerization of n-olefins. There is no teaching of enhanced selectivity or stability under catalytic cracking conditions. Nor is there any discussion of increased stability in rare earth exchanged SAPO.
SAPO catalysts mixed with zeolites (including rare earth exchanged zeolites) are known to be useful in cracking of gasoils (U.S. Pat. No. 5,318,696). U.S. Pat. Nos. 5,456,821 and 5,366,948 describe cracking catalysts with enhanced propylene selectivity which are mixtures of phosphorus treated zeolites with a second catalyst which may be a SAPO or a rare earth exchanged zeolite. Rare earth treated zeolite catalysts useful in catalytic cracking are disclosed in U.S. Pat. Nos. 5,380,690, 5,358,918, 5,326,465, 5232,675 and 4,980,053. The use of SAPO catalysts for cracking crude oil feed or xe2x80x9ccarbon-hydrogen fragmentation compoundsxe2x80x9d (materials with 5 or less carbons) is disclosed in U.S. Pat. Nos. 4,666,875 and 4,842,714 (SAPO-37 preferred for cracking gas oils). Although these patents disclose the use of rare earth exchanged SAPO catalysts, they state: xe2x80x9cAt present the presence of rare earth cations with the SAPO molecular sieves has not been observed to be beneficial to the activity of the SAPO component. The exact nature of the relationship of multi-valent cations and SAPO catalysts is not clearly understood at present, although in some instances their presence may be beneficial.xe2x80x9d (U.S. Pat. No. 4,666,875 at Col. 4 Lines 39-44, U.S. Pat. No. 4,842,714 Col. 11, Lines 29-34.)
The art has not previously recognized the highly selective conversion of hydrocarbon, especially naphtha feedstocks to propylene promoted by SAPO and related catalysts nor the improved stability obtained by rare earth exchanging such catalysts.
The invention provides a method for converting an olefinic hydrocarbon feedstock to propylene comprising: contacting a hydrocarbon feedstock under catalytic cracking conditions with a catalyst comprising a nonzeolitic catalyst selected from the group consisting of SAPO catalysts, MeAPO catalysts, MeASPO catalysts, ElAPO catalysts, ElASPO catalysts, rare earth exchanged catalysts from any of the preceding groups, and mixtures thereof, under cracking conditions to selectively produce propylene. Preferably the method is carried out to produce propylene in a propylene to ethylene ratio of at least 4:1 and a propylene to butylene, ration of at least 2:1. The invention further provides a method for stabilizing a catalyst from the foregoing group by ion exchange with a rare earth metal. A catalyst has enhanced stability as used herein when treated with a rare earth metal or metals in a concentration effective to provide a catalyst which exhibits a higher conversion of a hydrocarbon feedstock to propylene than does an equal quantity of an untreated sample of the same catalyst under the same conditions following exposure of each catalyst to steam for a period of at least 10 hours. The invention also provides an improvement in methods for catalytic cracking of an olefinic hydrocarbon feedstock to produce a light olefin containing product wherein it is desired to improve the propylene content of the product mixture. The improvement comprises mixing a catalyst selected from the non zeolitic catalyst group consisting of SAPO catalysts, MeAPO catalysts, MeASPO catalysts, ElAPO catalysts and ElASPO catalysts with a second cracking catalyst in a quantity sufficient to increase propylene content in the light olefin product while decreasing either ethylene or butylene when the product composition obtained with the mixed catalyst is compared to the product composition obtained with the second catalyst alone under the same reaction conditions.
The silicoaluminophosphate (SAPO) catalysts useful in the present invention have a three-dimensional microporous crystal framework structure of PO2+, AlO2xe2x88x92 and SiO2 tetrahedral units, and whose essential empirical chemical composition on an anhydrous basis is: m R:(Si[x]Al[y]P[z])O[2] wherein xe2x80x9cRxe2x80x9d represents at least one organic templating agent present in the intracrystalline pore system: xe2x80x9cmxe2x80x9d represents the moles of xe2x80x9cRxe2x80x9d present per mole of (Si[x]Al[y]P[z])O2 and has a value of from zero to 0.3, the maximum value in each case depending upon the molecular dimensions of the templating agent and the available void volume of the pore system of the particular silicoaluminophosphate species involved, xe2x80x9cxxe2x80x9d, xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d represent the mole fractions of silicon, aluminum and phosphorus, respectively, present as tetrahedral oxides, representing the following values for xe2x80x9cxxe2x80x9d, xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d.
When synthesized in accordance with the process disclosed in U.S. Pat. No. 4,440,871, the minimum value of xe2x80x9cmxe2x80x9d y in the formula above is 0.02. In a preferred sub-class of the SAPOs useful in this invention, the values of xe2x80x9cxxe2x80x9d, xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9din the formula above are set out in the following table:
Preferred SAPO catalysts include SAPO-11, SAPO-17, SAPO-31, SAPO-34, SAPO-35, SAPO-41, and SAPO-44.
The catalysts suitable for use in the present invention include, in addition to the SAPO catalysts, the metal integrated aluminophosphates (MeAPO and ElAPO) and metal integrated silicoaluminophosphates (MeAPSO and ElAPSO). The MeAPO, MeAPSO, ElAPO, and ElAPSO families have additional elements included in their framework. For example, Me represents the elements Co, Fe, Mg, Mn, or Zn, and El represents the elements Li, Be, Ga, Ge, As, or Ti. Preferred catalysts include MeAPO-11, MeAPO-31, MeAPO-41, MeAPSO-11, MeAPSO-31, and MeAPSO-41, MeAPSO-46, ElAPO-11, ElAPO-31, ElAPO-41, ElAPSO-11, ElAPSO-31, and ElAPSO-41.
The non-zeolitic SAPO, MeAPO, MeAPSO, ElAPO and ElAPSO classes of microporous, materials are further described in the xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d by W. M. Meier, D. H. Olson and C. Baerlocher (4th ed.; Butterworths/Intl. Zeolite Assoc. (1996) and xe2x80x9cIntroduction to Zeolite Science and Practicexe2x80x9d, H. Van Bekkum, E. M. Flanigen and J. C. Jansen Eds., Elsevier, N.Y., (1991).).
The selected catalysts may also include cations selected from the group consisting of cations of Group IIA, Group IIIA, Groups IIIB to VIIB and rare earth cations selected from the group consisting of cerium, lanthanum, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and mixtures thereof.
Preferred olefinic hydrocarbon feedstocks are nathphas in the boiling range of 18xc2x0 to 220xc2x0 C. (65xc2x0 F. to 430xc2x0 F.). The naphthas may be thermally cracked naphthas or catalytically cracked naphthas. The feed should contain from at least 10 wt % to about 70 wt % olefins, preferably 20 wt % to 70 wt %, and may also include naphthenes and aromatics. The naphthas may contain paraffins in the range of 5 wt % to 35 wt %, preferably 10 wt % to 30 wt %, most preferably 10 wt % to 25 wt %. For example, the naphtha may be derived from fluid catalytic cracking (xe2x80x9cFCCxe2x80x9d) of gas oils and resids, or from delayed or fluid coking of resids. The preferred naphtha streams are derived from FCC gas oils or resids which are typically rich in olefins and diolefins and relatively lean in paraffins.
Catalytic cracking conditions mean a catalyst contacting temperature in the range of about 400xc2x0 C. to 750xc2x0 C., more preferably in the range of 450xc2x0 C. to 700xc2x0 C., most preferably in the range of 500xc2x0 C. to 650xc2x0 C. The catalyst contacting process is preferably carried out at a weight hourly space velocity (WHSV) in the range of about 0.1 Hrxe2x88x921 to about 300 Hrxe2x88x921, more preferably in the range of about 1.0 Hrxe2x88x921 to about 250 Hr1xe2x88x92, and most preferably in the range of about 10 Hrxe2x88x921 to about 100 Hrxe2x88x921. Pressure in the contact zone may be from 0.1 to 30 atm. absolute, preferably 1 to 3 atm. absolute, most preferably about 1 atm. absolute. The catalyst may be contacted in any reaction zone such as a fixed bed, a moving bed, a slurry, a transfer line, a riser reactor or a fluidized bed.
Test Conditions
A series of runs in a small bench reactor was conducted on hexene as a model compound. Comparison runs with a ZSM-5 zeolite catalyst commercially available from Intercat. Inc., of Sea Girt, New Jersey were conducted over a fixed bed of catalyst. The effluent stream was analyzed by on-line gas chromatography. A column having a length of 60 m packed with fused silica was used for the analysis. The gas chromatograph was a dual flame ionization detector equipped Hewlett-Packard Model 5880. All tabulated data is in weight per cent unless otherwise indicated.